Conform extrusion is a metal extrusion process in which the force for extrusion of the metal material through a die is derived, at least in part, by maintaining frictional engagement of the metal material with passageway defining surfaces of a member which is moved towards the die such that frictional drag of the passageway defining surfaces urges the metal material through the die. Apparatus for performing this method is disclosed in U.S. Pat. No. 3,765,216 ("'216") to Green and assigned to the United Kingdom Atomic Energy Authority.
The '216 patent describes an extrusion apparatus comprising a wheel member having an endless peripheral groove therein and a fixed shoe member covering at least part of the length of the groove which forms a passageway therewith. An abutment member projects from the shoe member into the groove and blocks one end of the passageway. The wheel member is rotatable relative to the shoe member in the direction towards the abutment member and at least one die orifice is associated with the abutment member.
The metal feed material to be extruded is introduced into the end of the passageway at a location remote from the abutment member and the frictional surfaces formed by the peripheral groove in the wheel carry the metal material to the abutment member. The resulting frictional forces provide a bulk compressive stress applied in the metal material to be extruded so as to feed the material into the region forward of the working face of a tool member which contains the die orifice. The bulk compressive stress forces the metal material through the die to form the conformed extrusion product.
U.S. Pat. No. 4,552,520 ("'520"), to East et al., and assigned to Metal Box Public Limited Company, discloses that a loose particulated or a comminuted form of metal material as feedstock may be supplied to produce an extrusion which closely resembles that achievable with feedstock in solid form, provided the groove includes tooth members on one or more sides of the frictional surface-forming peripheral groove which match oppositely disposed corresponding tooth members on the opposite side of the groove to remove undesirable flash. However, it has been found that particulated material, such as powdered metal, may not always flow smoothly and uniformly through the groove. The particulates have no structural integrity; regularity of flow into the Conform machine is thus permitted to become uneven. The particulated material is subjected to flow turbulence and becomes less uniform due to the mixing and shear forces across the material flow passageway to which the feed material is subjected. This is, of course, a serious problem which heretofore has limited the extrusion of powdered metal in Conform machines.
A particular problem with prior art zone melting and melt texturing production methods of certain high conductivity materials is that these processes require extremely long-term annealing periods (e.g., 150 hours) and are capable of producing products which are necessarily short in length, while normal high-speed production methods of producing conventional conductors is incapable of producing these superconductors.
The advantages of the Conform extrusion machine over conventional extrusion apparatus include the provision of a theoretically continuous extrusion process, with attendant simplification of subsequent handling techniques and the elimination of billet discards. Examples of prior art Conform extrusion apparatus of the aforementioned type are also described in U.S. Pat. No. 4,055,979 to Hunter et al.
Considerable heat is generated by the enormous frictional resistance and resulting axial stress encountered by the feedstock as it is carried along the groove by the rotating wheel, as a consequence of the close contact of the latter with the extrusion shoe. In a typical Conform extrusion process, an expansion chamber may be provided in the extrusion shoe, located adjacent the blocking abutment and upstream of the die, to allow extrusion of product having cross-sections other than that of the feed material.
The shearing forces in the feed material are higher along the extrusion shoe which is fixed relative to the moving material than along the grooved rotating wheel with which the material is moved. Thus, it may be necessary to apply differential cooling about and along the extrusion path axis. In a typical process, the extruded product may be fed into a water-quench tank located some distance from the exist die. It has been found that such prior art Conform machines produce extruded products which may be subject to undesirable characteristics.
The prior art Conform machines have been found to be limited in their ability to accommodate different feedstock materials and to produce unique propertied extrudates having special characteristics. The present invention includes the addition of a device to compress the feed materials to coalesce or agglomerate in a compacted form as the Conform feedstock. The feed materials are compacted sufficiently to cohere and maintain a generally fixed shape, thus enabling smooth and uniform flow into the extrusion passageway of the Conform machine. The feedstock materials accommodated by this compacting function include powdered and particulated materials and material mixtures having widely varying melting and solidification point temperatures.
In order to provide for compression of powdered or comminuted metal material feedstock as it enters the Conform wheel extrusion process, the present invention incorporates a plurality of peripheral wheels having metal forming surfaces which cooperate with a plurality of shoes to form the unique extrusion product. It has been found that the powder material can be compressed to about 40 percent in a preliminary step. However, this compacted material may not be completely solid and therefore may require a secondary shoe for guidance into the passageway of the Conform extrusion wheel.
More specifically, the improved apparatus includes a forming roll cooperating with both an auxiliary shoe and with a grooved Conform wheel. The feedstock is supplied at the juncture of the forming roll and the Conform wheel. The forming roll exerts compressive pressure on the powder feed material to compact it, essentially forming a preform feedstock. The first shoe, here called the secondary shoe, is positioned to direct the compacted powder material feedstock into the Conform machine.
The auxiliary shoe includes a tapered blade edge which acts as a "doctor blade" or stripper member to remove the initially compressed powder feed material from the first wheel and direct it into the second (Conform) wheel. A more conventional extrusion shoe cooperates with an abutment member in the Conform wheel peripheral groove and with an extrusion orifice upstream of the abutment, to extrude the compacted feedstock.
This improvement is particularly suitable for the extrusion of very fine particles of superconducting powders and for aluminum alloy powders. With this apparatus, certain special alloys can be produced, in the case of some materials without requiring the addition of a binder material. Examples of such compounds include yttrium, barium, and copper oxide (so-called "1-2-3 compound") which has a melting point of from about 1020.degree. C. to about 1050.degree. C.; bismuth (Bi), strontium (Sr), calcium (Ca), and Copper Oxide (CuO.sub.2) (so-called "1112 compound"), having a melting point of from about 895.degree. C. to about 900.degree. C.; and silver (Ag) powder having a melting temperature of about 960.5.degree. C.
Additionally, other yttrium-based compounds, other bismuth-based compounds, and thallium-based compounds may also be used as feed materials. A combination aluminum, vanadium, iron, and silicon alloy powder feed has been used as a feed material to produce small cross section extrudate rods. Other special alloys may be extruded from powdered or particulated materials, including high-strength rivet stock.
Powder-sintered form high-Tc superconductors can ordinarily carry only low transport critical current density (Jc) unless produced with highly textured microstructures, which are difficult to achieve, but which may be produced by zone melting and melt texturing, a process which requires an extremely long-term annealing period as described. These products are necessarily short in length. The present invention is expected to produce a highly textured microstructure high-Tc superconductor without the expensive, time-consuming zone melting, melt texturing, and long-term annealing, as in the prior art.
The method and apparatus disclosed herein provides a number of advantages in producing these and other unique extruded products. Among these advantages are the fact that the extrusion products will have a density close to the theoretical density. During extrusion, the material is known to become plastic but does not melt completely. With many of these unusual alloys, this effect tends to keep the "1-2-3 compound" in one phase; the result is improved extrudate properties.
The extrusion product grains may be aligned in the extrusion direction; this is known to produce unique properties in some materials, as was found in "Critical Currents in Silver Sheathed (Bi, Pb).sub.2 Sr.sub.2 Cu.sub.3 O.sub.10 Feed Produced by Superconducting Tapes," by Donglu Shi et al., and in "High Critical Current Density in Grain-Oriented Bulk YBa.sub.2 Cu.sub.3 O.sub.x Processed by Partial-Melt Growth," by Donglu Shi et al., Applied Physics Letters, July 1990. The resulting extrusion product will be in an annealed condition. Further in-line processing may be adapted to include wire drawing, oxygen or other annealing, and other downstream processing steps.